1. Field of the Invention
The present invention relates to positioning apparatus, more particularly programmable robotic apparatus where precise positioning of a manipulator means relative to a workpiece or reference is required. More particularly the invention relates to a movable stage that can be attached to and carried by the manipulator means of a conventional robot which enhances the operation of the overall combination by overcoming the inherent position inaccuracies of a relatively massive machine required to displace work holders to large distances.
2. Description of Prior Art
As more and more industrial robots find their way to the manufacturing floor, it is becoming evident that two major disadvantages are lack of speed and positioning accuracy which in many cases are universally related. While great advances have been made in their "intelligence" and electronics, very little has been done to improve accuracy.
Although robots provide greater versatility and large range of motion, they cannot compete with hard automation when it comes to accuracy and speed. If a robot is made stiffer and heavier for increased accuracy, speed is usually sacrificed and vice versa. While many robot manufacturers boast of their high repeatability, they rarely release any accuracy specifications. Needless to say, accuracy is not very good. Many are only accurate to within one-hundred thousandth of an inch.
Repeatability and accuracy are distinguished in the following manner: if a robot is physically moved to a point in space and consequently taught that point, it would be capable of always returning to the same point within a given tolerance known as "repeatability specification". On the other hand, if the robot is programmed to move a calculated destination in its coordinate system, the deviation from the command position is referred to as "accuracy".
Most commercially available manipulators receive position feedback from encoders or potentiometers located at each joint. Thus the absolute end point position is derived from various joint displacements. If there is a slight measurement error associated with the encoders, it is greatly amplified through the length of each link, resulting in a large end point error.
Another cause of positioning errors is beam bending. When a link is subjected to external loads or inertial forces, it usually bends, causing the end point to deviate from the predicted value. Beam bending gives rise to both static errors and dynamic oscillatory errors.
There are many industrial operations in which success relies on the high repeatability of the robot. Several critical points are taught the system and the manipulator repeats the task time and again. This is practical only when there are a few points to which the robot must travel, as in stacking parts on a common axis. Nevertheless there is an enormous amount of applications where high accuracy is necessary to accomplish the job. In drilling and riveting applications, parts must often be manufactured according to a print with numerous holes dimensioned from a certain reference line. In a flexible manufacturing system with the intended capability of efficiently handling small production runs, it is impractical to re-teach numerous points each time the part changes. The position of these holes are programmed into the computer, and the robot is commanded to move to those points to within a slight tolerance. Thus, high accuracy is a must.
One way of reducing bending in a link is by increasing the cross-sectional moment of inertia. The easiest way of doing this is by simply increasing the cross-sectional area; in other words, using a thicker link. Unfortunately, this implies a larger mass and speed is greatly sacrificed. One technique for increasing the accuracy of a robot that is designed for a large range of motion in which great bending moments are created is to provide a local support in the vicinity of the task location. Prior to beginning the operation, the robot would attach itself to the support and thus obtain a reference location close to the work operation. However, the support theory has disadvantages. A local support must be provided for each point where an operation is to be performed. If there are numerous such points, it becomes extremely difficult to carry out this mode of operation. Further, different supports must be provided for different production runs.